add_filter( 'vc_grid_item_shortcodes', 'my_module_add_grid_shortcodes' );
function my_module_add_grid_shortcodes( $shortcodes ) {
    $shortcodes['vc_post_content'] = array(
        'name' => __( 'Post Content', 'my-text-domain' ),
        'base' => 'vc_post_content',
        'category' => __( 'Content', 'my-text-domain' ),
        'description' => __( 'Show current post content', 'my-text-domain' ),
        'post_type' => Vc_Grid_Item_Editor::postType(),
    );
    return $shortcodes;
}

// output function
add_shortcode( 'vc_post_content', 'vc_post_content_render' );
function vc_post_content_render() {
    return '<p>{{ post_data:post_content }}</p>'; // usage of template variable post_data with argument "post_content"
}